1. Field of the Invention
The present invention relates to a method of soil decontamination and more specifically to a method of soil decontamination induced by electric fields.
2. Description of Related Art
Electroosmosis is the phenomena of migration of an ionic liquid through a porous charged surface under the action of an applied electric field.
Electroosmosis has been applied to reducing the water content of soil and consolidating the soils for construction purposes as described in Bjerrum, L., et al., Geotechnique, 17, P. 214-235 (1967) and Casagrande, L. J., Boston Civil Engineers, 69 (2), P. 255-302 (1983).
Electroosmosis has also been applied to reducing the water content of mine tailings and waste sludges as described in Lockhart, N. C., Colloids and Surfaces, 6, P. 229-251 (1983) and Lockhart, N. C., Int. J. Mineral Processing, 10, P. 131-140 (1983).
U.S. Pat. No. 4,783,263 Detoxification Process by Trost, P. B. and Rickard, R. S. issued Nov. 8, 1988 describes a method of soil decontamination that comprises collecting contaminated soil, mixing it into a slurry and adding surfactants or alkaline agents to the slurry. This is referred to as a batch process. The surfactants or alkaline agents are used to concentrate the contaminants into a liquid phase. The liquid phase is then removed with the contaminate. This method is not an in-situ method since it involves digging up the contaminated material before processing it.
FIG. 1 illustrates principles of electroosmosis. During electroosmosis an electrolyte solution 4, such as an ionic water solution, moves through a plurality of pores 6 relative to a surface of a porous media 2 such as soil, clay, sand or other mineral particles under the influence of an applied electric field. Typically, a porous medium 2 accumulates a number of negative charges 8 at the surfaces in contact with electolyte or ionic solution 4. These charges attract a number of positive ions 10 in solution 4 and repel negative ions 12. Positive ions 10 will therefore predominate in the layer of ionic solution 4 next to the surface of porous medium 2. The electrostatic interaction between the charges 8 in the surface and the ions 10 in the water produces an "electric double layer", in which the surface charge of the porous medium 2 is balanced by an adjacent layer of charges 10 of opposite sign in ionic solution 4. In typical soils saturated with aqueous solutions, the double layer is very thin, on the order of 1-10 nanometers as described in Probstein, R. F., et al., Chemical Processing, 11, P. 35-40 (1990).
A static electric field is applied to the soil having electric field lines in the pore in the direction shown by arrow 14. The static electric field is established in the soil by applying a D.C. voltage across a pair of electrodes, anode 16 and cathode 18. The electric field exerts an electrostatic force on the charged double layer, causing ionic solution 4 to move in a direction parallel to the electric field. Ionic solution 4 migrates toward and accumulate near one of electrodes. The accumulated ionic solution is then removed by some appropriate means such as pumping.
The bulk of the liquid in the pores far from the double layer is set in motion by viscous interaction with ionic solution 4 near the double layer. R. F. Probstein, in Physicochemical Hydrodynamics: An Introduction, p. 192, Butterworth (1989) showed that the induced liquid velocity, v, is given by the expression: ##EQU1## where .epsilon. is the electric permittivity of the liquid, .mu. is the liquid viscosity, .zeta. is the surface potential of soil corresponding to the charge it accumulates, and E is the applied electric field. The induced liquid velocity, v, increases with increasing electric field, but decreases as the liquid becomes more viscous. Since liquid movement is parallel to the electric field lines, the direction of flow is highly controllable with the electroosmosis process.
Another advantage of the electroosmosis process is that the flow distribution 19 of FIG. 1 is uniform, being essentially a plug profile. A plug flow distribution is characterized by low fluid flow velocity near the surfaces, while the flow velocity is constant across a large area in the center of the pore. Plug flow distributions are independent of pore size and uniformity of pore sizes as described in Probstein id. at p. 35-40 (1990). Due to plug flow, electroosmosis can be effective with low-permeability soils and clays and soils that are not very porous.
The removal of chemical species from porous media by means of electroosmosis relies on the movement of the pore liquid containing the contaminant species toward one electrode where the liquid is collected. A purging liquid is introduced to maintain soil saturation to prevent crack formation as the liquid within the soil is removed. The purging liquid also preserves the effectiveness of electroosmosis. For further detailes see Shapiro, A. P. et al., Solid/Liquid Separation: Waste Management and Productivity Enhancement, H. S. Muralidhara, Ed., P. 346-353, Battelle Press, Columbus, Ohio.
Efforts to apply electroosmosis techniques to the removal of contaminants from a waste site are relatively recent. Renaud and Probstein carried out limited laboratory studies on the use of electroosmosis for the removal of acetic acid and phenol from saturated clay samples. These are described in Renaud, A. P., et al., J. Physico Chemical Hydro., 9, P. 345-360 (1987). Their work showed that electroosmosis might be particularly useful in control and remediation at hazardous waste sites.
Removal of contaminants from the soil is further complicated when the contaminant is not soluble in the electrolyte solution. As is commonly the case, in-situ contaminants are trapped in soil having a considerable amount of ground water. Many of the hazardous contaminants are not water soluble, and removal of the contaminants is very difficult.
Currently there is a need for a method of removal of contaminants which are not water soluble in-situ. Also, there is a need to control the direction of seepage of contaminants imbedded in the ground.